The invention relates generally to a safety system for a steam turbine installation. The safety system is based on the closed-circuit principle and includes a two-channel low-pressure section, in which safety devices are incorporated, and a high-pressure section.
Safety systems based on the closed-circuit principle are known in the field of control systems for turbines. Furthermore, the normal control system of a turbine functionally falls into two parts, namely the control system itself and the safety system. The safety system is intended as a second line of protection against unacceptable operating conditions, such as overspeed, loss of vacuum, etc. As its most important components, the safety system includes main stop valves, which are usually high-speed stop valves, that shut off all supply of steam to the turbine in the event of disturbances.
A known safety system includes a two-channel, low-pressure section, having an emergency oil system provided with safety devices, and a high-pressure section which is either an oil system or a control-fluid system. The emergency oil system is in the form of a primary system in the sense that in the event of disturbances the oil system is directly de-pressurized by various safety devices. The oil system operates with low-pressure lubricating oil and is divided into two parallel channels, because the safety devices connected to the oil system must be tested while the steam turbine installation is in service. During testing, one of the channels is disconnected from the oil supply and the turbine continues to be protected by the devices in the channel not being tested, since the safety devices are duplicated.
The high-pressure section has a single-channel configuration and includes, among other things, a number of de-pressurizing relays. Hence, the high-pressure section operates solely on the discharge-control principle. With a configuration of this kind the functioning of the relays cannot be checked while the steam turbine installation is in service since the safety system has to be operational by way of the single channel. Furthermore, with a pure discharge control there is a continuous supply of operating medium, and therefore the pressure cannot drop abruptly to zero when required.
An object of the present invention is to create a safety system in which all of the components involved, apart from the unavoidable manual trip, which normally acts simultaneously on both channels of the low-pressure system, can be tested during service with respect to their operational status.
In the present invention, a safety system based on the closed circuit principle and having a two-channel low-pressure section provided with safety devices and a high-pressure section has each channel of the low-pressure section acting hydraulically on the high-pressure section by way of a tripping and reset relay. The high-pressure section is supplied by way of a high-pressure hydraulic system and is divided into an intermediate safety system and a principal safety system, both of which have a two-channel configuration. The high-pressure section is further arranged into a one-channel common safety system via which the stop valves of the steam turbine installation are actuated. Feed and discharge amplifiers are connected in series hydraulically with respect to their supply and separate the intermediate safety system from the principal safety system. A separating relay is located between the principal safety system and the common safety system in such a way that the safety devices in the low-pressure section can be tested channel by channel while the steam turbine installation is in operation. During testing the series arrangement of the feed and discharge amplifiers is interrupted.
An advantage of the present invention is to be seen particularly in the fact that an almost completely safe system can be constructed at a relatively small cost. The series arrangement of the feed and discharge amplifiers causes the operating pressure in the system to fall abruptly and advantageously to zero in the event of a turbine trip.
It is convenient if all of the components of the high-pressure section are contained within a control box. Furthermore, it is desirable to have the feed and discharge amplifiers and the separating relay also contained within a common housing. The first of those arrangements has the advantage especially in the case of steam power plants with boiling water reactors that the entire control box can be situated outside of the "hot zone" (zone of dangerous radiation). Accordingly, the control box is easily accessible. The second arrangement permits an inexpensive, easily maintained construction whereby substantial savings in sealing points and in possible sites of leakage can be achieved.